I'd lost touch with Nigella over the years. Back in the 80s, she was a young thing out of Oxford precociously in charge of the Sunday Times book pages and I was an ardently feminist novelist. Things have changed: now she is the millionaire queen of cookery and I am a less ardent feminist. Now here she is again - sitting next to me on the Eurostar to Paris. At 48, she is shockingly beautiful. It's the white translucent skin that does it; the full, sensuous lips, the slightly slanty eyes, the thick, black hair. If she's changed from those early years, it's that she now seems outlined with a heavier pen than she once she: there is no melting her into the crowd any more. Even the thoroughly decent black woollen dress she wears doesn't help.

All the same, she seems to have little interest in how she appears to other people. She never watches her own shows, switches off the radio if she hears her voice, doesn't even look in the little viewer on digital cameras to see what the photographer sees. "I am not a mirror person," she says. She only ever looks at herself a small section at a time, and then only if she has to, as when doing make-up. "Why would you want to look in the mirror? If you feel good about yourself, go on that, because you're never going to look as good as you feel. And when you feel bad about yourself, you don't really want to have any evidence."

The image people have of Nigella is, of course, quite different: she's the woman who can look sexy in the kitchen, even with her hair straggly from cooking steam, up to her elbows in icing sugar. Her cleavage looms large in the public imagination. Men like to look at her, women want to be her. She's the woman who has it all - the career, the children, the looks, a rich art dealer for a husband, her own TV show. She is a media celebrity, much photographed, everything from the death of her first husband to her waistline pored over by the press. She loves food, doesn't torment herself dieting, and mostly gets away with it. Sometimes it shows, and sometimes it doesn't. Today on Eurostar it doesn't: her chin is well defined; her eyes large and luminous. But then she's not in anxious mode.

"I'm always so envious of those people who don't eat when they're anxious. I only have to be a little anxious and I overeat. It's like someone's taken a bicycle pump to me and I inflate. When my mother died, everyone said, 'You look very well' and I felt terrible, but I looked, um, round-faced."

She swears it's the strain of making the TV shows as much as all the calorific tasting and trying that puts on the weight.

"I've had the same TV crew for ever. I'm very family-minded, so they become like my work family. The director once did a weigh-in, an every Monday morning weigh-in. For a while, it was neck and neck between me and the sound man as to who had put on the most weight. In the end I won."

Eurostar food arrives, surprisingly limp considering we're not even pressurised, on an airline-style pre-set tray. From Nigella, there is polite disdain: she's had her usual porridge for breakfast and then some rye bread dipped in walnut oil with a smear of Marmite before she left home. The tray is sent back. Instead, she brings out a plastic box of homemade cookies - Christmas Rocky Road - I later find them on page 28 in Nigella Christmas, the book she is currently promoting. A mixture of dark and milk chocolate, brown and white sugar, glacé cherries and brazil nuts, baked and dusted with icing sugar. Delectable. It's the dusting of icing sugar that gets me. She has bothered to find the sieve, find the icing sugar, and then presumably cleared up the bits that drift away to cause stickiness wherever they fall. She cares what food looks like as well as how it tastes. I feel I am rather like Nigella's mother, the Lyons heiress Vanessa Salmon, who made things taste nice but didn't bother with appearance and never consulted a cookbook in her life, and then went off and married Freddie Ayer, the philosopher, while her father (whom Lawson once eschewed but nowadays loves dearly) stayed home to become Mrs Thatcher's chancellor.

She talks about her childhood: how she couldn't wait to grow up. How she read and read and read while she waited. "My parents were very young, and more interested in each other than in us." I point out that when she was small, this was normal: if you asked parents which they would save from drowning, the children or the spouse, the answer came back, "The spouse. You can always have more children." These days they say, "The children. You can always find another spouse." But Nigella is not comforted. "At least there were four of us," she says. "Large families are good; you can talk about how much you hate your parents instead of bottling it up."

She would have liked more children, but it didn't turn out like that. She was married to the journalist and writer John Diamond, and they had Cosima and Bruno - now 15 and 12 - and then John got ill and died over three years, very publicly recording his death from cancer in a newspaper column. Nothing private about that. I decide this is how and why she manages to write as she does - the prose lucid and companionable, persuasive and endearing. Nothing cures grief, but you might as well try. You can't stand and stare into space for ever after the worst has happened. You just have to keep busy. You bring out another book, do another show.

Diamond's death followed the premature deaths of her mother and sister, Thomasina, also from cancer. Grief is never over, she maintains. "There is something quite contemptible about the human spirit; the way human beings carry on with life in the attempt to make everything normal ... otherwise of course you couldn't carry on. And yet I don't admire that. It's a very base instinct. It isn't normal. The question I always hate is when people say, 'How's she coping?' What does it mean? Everyone copes on the outside but not on the inside.

"I don't believe in crying out loud," she says. "Everyone is meant to now. I hate the grief police. I hate the way they try to make you cry. I didn't want that. I don't know what's more annoying, people who cross the street not to see you because they're embarrassed or people who try and make you cry by putting their arms around you."

She describes herself as cynical; she knows what people really think and what they are - whatever they say. She attracts envy, spite, malice: she knows it. But not, she says, from people she cares about.

She tells me she was moved from school to school when young. She was never expelled, but she argued with everyone: she could never accept authority. She went through a stage of being religious: "I was about 14 or 15, and I wrote an essay - 14, 20 pages; long, anyway - about Jesus and I got it out of my system. I would like to be religious and I sometimes think that but I just don't have the genes for religion. But I might find God later. Who knows?"

She says she doesn't actually like people, but "I'm interested in them. And I'm nosy, very nosy. I want to know everything about everyone. When I was a waitress, what was so fantastic was hearing snippets of people's conversations." But I think, in truth, she likes people well enough, certainly enough to want them to enjoy what she cooks. She follows my progress with the Christmas Rocky Road carefully enough. She tells me she liked working in Harrods in her student days, going in the staff entrance, and the feeling of belonging it gave her. "Mind you," she adds cautiously, "I've never been a great believer in belonging. When I was at Oxford, I never felt fully paid up to any particular group." Not that she'd suffered from it. "It gave me huge superiority. I quite liked being perfect friends with the posh lot and perfect friends with the, well, not exactly nerdy - me being a linguist rather than a mathematician - but the ones who just wanted to go into the library and work." She pauses, puzzled. "Some people I was at university with have never made friends with anyone outside since. They've even married someone from there. They've kept the same circles. I find that very odd."

When she was younger, she wanted to write the great novel of the 20th century, but there was something "very exposing" about that, so she began reviewing books instead. It was only when Charles Moore, then editor of the Spectator, asked her to write about something else that she first stumbled into food. A line she'd once read had stuck in her head. A young man, given champagne, described it as "tasting like apples peeled with a steel knife". No, she couldn't remember the exact novel, but that precise evocation of the drink's metallic acidity wouldn't go away and persuaded her it could be really interesting to write about food. So she became the magazine's restaurant critic.

Across the train corridor, two men are trying to appear to be trying not to listen. One of them is a fresh-faced lad from Wales, the other an elderly, mid-European intellectual wearing Armani glasses. The train stops. We are in the Channel tunnel. This is not good. I tell her I carry diazepam in case of terrorist attack: would she like some? She offers me Xanax. The elderly intellectual offers us something he has from his doctor in France. The man from Wales looks horrified. We all decline each other's offers. The train moves on. The elderly intellectual is clearly in love with Nigella and would marry her if he could. She offers him some Chocolate Rocky Road from the Tupperware box and he accepts.

I ask her about all those media reports and general outrage about her saying she wouldn't leave any money to her children. She dismisses it. She was misreported. "I would never discuss my will. What I said was that I didn't want to give them money while I was alive because I worked hard and see the benefit, and I try not to spoil my children."

She tells me she hated being a child. "I did feel very guilty having children. I thought: how can I inflict childhood on them? I thought I was joining the enemy camp by becoming a parent."

Her own family life now with Charles Saatchi is pretty quiet. "I don't really go out very much. A friend, who'd had children before me, told me that you can have two lives but not three. You can work and have children, or you can go out and have children, or you can go out and work, but you can't do all three. And I don't think you can, really. I've never recovered from that tiredness you get when you have babies. When you become obsessed by what time you go to bed and what time you're getting up."

Is she a feminist? "I think I still feel annoyed that the male perspective is thought of as the general view of things. I feel that, as a woman, you spend a lot of your life being patronised by stupid men who think that they're cleverer than you just because they're men."

Nigella goes to the ladies' and offers me a battered Elinor Lipman volume of short stories to read in her absence. She still has good taste in fiction. On her return, she says she went back to Oxford the other day and was seized by melancholy. The elderly intellectual interrupts and denies Oxford is in the least melancholy. He is a professor there. He should know. He tells her he has houses in Oxford, London, Vienna, Paris - it sounds like a proposition. "Yet what I like about men," Nigella says, aside, "is the assumption they all make that they're riveting. Extraordinary. But then I suppose we women just assume we're captivating."

We part on the arrivals platform. Nigella puts on her beret and looks very cute, young, defenceless and, suddenly, French. There is no one there to meet her. I have to go. "You're not even going to try the hot sugared doughnuts they have on the stall?" she asks. I shake my head. Her minder turns up, breathless. Nigella smiles. "If you just stand, they always come."

• Fay Weldon's latest novel, The Stepmother's Diary, is published by Quercus at £16.99